Adhesion additives are known throughout the automotive coatings industry. As is understood by those skilled in the art, adhesion additives are used as components in primer surfacers, or other intermediate coating compositions, to promote adhesion between a substrate and a topcoat system for an automobile, such as a topcoat system including a flexible basecoat and flexible clearcoat. In general, plastic substrates may be coated with curable, or thermosettable, coating compositions. Color-plus-clear composite coatings have been particularly useful as topcoats for which exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. Adhesion additives are primarily used in primer surfacers, typically solventborne primer surfacers, that are applied to a bumper, i.e., facie, or other trim component as the substrate. Typically, these substrates are made up of thermoplastic polyolefin (TPO), and without the inclusion of an adhesion copolymer in an intermediate solventborne primer surfacer layer, the topcoat system may delaminate from the TPO substrate.
One example of an adhesion additive is chlorinated polyolefin. Other adhesion additives are olefin-based based copolymers that have an olefin block that is substantially saturated and at least one (poly)ester or (poly)ether block. The olefin-based block copolymer is typically present in an organic solvent such as xylene, toluene, and the like. The individual components of the adhesion copolymers, i.e., the olefin-based block copolymer, frequently settle out into the organic solvent. This settling renders the adhesion copolymer unstable, i.e., having poor shelf stability, and therefore, not suitable for use as a component of a solventborne primer. These are described in U.S. Pat. Nos. 6,300,414 and 4,898,965. These patents, however, do not include chlorinated polyolefin as an adhesion additive and do not teach the method of the present invention of hot blending chlorinated polyolefin and the olefin-based copolymers and coatings containing adhesion additives prepared thereby.
Adhesion promoters including chlorinated polyolefin and a diene that is not reacted with epsilon caprolactone are taught in U.S. Pat. No. 5,863,646. However these coatings demonstrate less effective adhesion than olefin-based copolymers combined with chlorinated polyolefin prepared according to the method of the present invention.
It would be desirable to provide a coating composition comprising a more stable adhesion promoter that provides improved physical properties, including improved adhesion under harsh testing conditions.